Mechanical energy can generally be resolved into components of force and velocity. In the case of rotational force, these are in the form of rotational velocity (RPM) and torque. Power transmissions typically convert energy from one speed or range of angular velocities and range of torque to output energy at a different range of angular velocities. Conservation of energy dictates that the input speed and torque resolve to the same energy as the output energy, minus energy conversion losses.
Such energy conversion losses usually result from the attempt of the power transmission to accommodate a wide range of angular velocities while still converting the energy efficiently. Normally an operational speed output range is accommodated, along with a low speed for a startup or stall. If lower than normal operational speed or stall speed output is accommodated along with a significantly high input speed, the efficiency is compromised. Accommodations of this discrepancy are well-known when associated with land vehicle transmissions.
In a fluid clutch transmission, high stall speeds permit maximum initial torque output, but compromise overall efficiency. This type of device does a poor job of matching the output load to the prime mover over a wide range of input speeds.
Mechanical clutch transmissions, on the other hand, rely on slippage in order to accommodate low velocity starts from stall velocity. Maximum torque output at stall results in excessive clutch wear. While tolerable in racing applications, this is not acceptable where routine operation requires longevity of operating components. Again, there are no provisions to match the output load to the input power.
Continuously variable speed drives are capable of small variations in input to output speed ratios, but also have difficulty in handling stall velocities. In addition, wear characteristics of variable speed drives are often unacceptable. Power capabilities and efficiencies are usually low.
It is not necessarily desirable that input speed be maintained at a constant rate, even in the case of continuously variable speed drives. In many prime movers, the input speed increases proportionally to output power, but only within a narrow range as compared with output speed. This is typically the case with Otto cycle internal combustion engines, diesel engines, and human power as applied to a bicycle. Narrower speed ranges are common to some electric motors and gas turbine engines. Accordingly, it is desired to have a power transmission which accommodates a wide variety of output angular velocities but does not excessively restrict variation in input angular velocities.
In the case of a power transmission used on a bicycle, if a relaxed cadence is applied, the input force should not be excessively high. If a sprint cadence if applied, the rider may prefer that this force be slightly different. In this manner the cyclist is able to sustain high power output resulting from a fast cadence. In an ordinary gear change transmission (a derailleur and sprocket arrangement on the bicycle), the rider selects a gear which accommodates the desired range of force and cadence.
Stall speed is normally not a problem on a bicycle, but at slow speeds encountered on steep hills, it is desired that a transmission quickly adjust to the ideal ratio of cadence to wheel speed. This is difficult with standard hub type or derailleur type gear change transmissions because, during the gear change, the load must be interrupted or reduced and because the gear changing operation occurs relatively slowly.
An additional disadvantage of gear change transmissions is that the operator must have an understanding of the relative speeds of input power and output power. In some cases, and for some people, this comes naturally, but in other cases, this determination can be tasking.
If a motor is substituted, the motor must be able to run at idle, and with application of power be able to speed up to near its maximum output speed with the transmission's output speed at stall. Input speed should then be able to vary along a desired range in proportion to output power, in order to maintain a desired input speed at each power setting.
In the case of electric motors, low speed operation may create stress on the motor. In some applications, such as compressors or pumps which are re-started with a significant output pressure head, the load on the motor may prove to be excessive. Typical applications where this is a problem are air conditioner compressors, where delay start or thermal shutoff circuits are used to protect the motor. Ideally, the motor should have an ability to re-start with a minimum load so that the motor can accelerate to an ideal operating speed.
It is therefore desired to provide a power transmission which is adaptable to a wide variety of output speed conditions, and which accepts input speed and power and be able to generally track an ideal power curve for the input power source or prime mover.
It is further desired to provide a power transmission which has good output stall characteristics in that input speed may be maintained during output stall without undue wear on the transmission and without significantly impeding output efficiency. Ideally, power consumption of the prime mover during output stall conditions should be minimized in order to reduce loads on the prime mover and in order to permit the prime mover to deliver a high output power when the output is accelerating at near stall speeds.
It is desired that the power transmission be able to provide an output speed which ranges from stall to a maximum speed which can be anticipated given the power consumption of the output device and practical limitations. The ratio of input speed and power should closely approximate the capabilities of the input device or prime mover, except at stall, where low power consumption at idle should be accommodated. Ideally, power losses in the transmission should be minimal, so that maximum output power and maximum power efficiency be achieved.